1. Field of the Invention
This invention relates to the production of safety glass, and in particular to production of multi-layer glass-plastic laminates formed in curved or spherical shapes, for use in such products as face masks, windshields, etc.
2. Description of the Prior Art
Laminations of glass and plastic have been known to be useful as safety glass for some time. In general, a glass layer is employed for resistance to scratching and weathering. Although unlaminated glass could be provided in thicknesses sufficient to preclude shattering, such a glass would be unacceptably heavy and noticeable defraction would detract from the optical properties of such a product. A thinner glass part can be combined with a plastic layer to provide a lighter weight product having acceptable scratch and shatter resistance. If the bonding between the glass and plastic is adequate, upon impact the glass will shatter into many small parts, a large proportion of which will cling harmlessly to the plastic. Moreover, in a glass-plastic product, the plastic layer serves as a barrier to protect the users eyes from shattered glass.
In the prior art, curved safety glass forms have been produced from a sandwich of clear glass around a polycarbonate sub-layer. Generally, although not always, this bonding was achieved by use of polyvinyl butyral. An example of a flat configuration, using five laminated layers including a polyaryl carbonate layer, is disclosed in U.S. Pat. No. 3,388,032--Saunders. Saunder's safety glass consists of respective layers of glass, polyurethane, polyaryl carbonate, polyurethane, and glass. A second example, U.S. Pat. No. 3,666,614--Snedeker, et al uses an ethylene-vinyl acetate copolymer for adhesion between the glass and polycarbonate, and promotes adhesion using organo-silicon compounds incorporated into the copolymer. Both patents teach processing at elevated temperatures in the range of 392.degree. F. (200.degree. C.).
An alternate product to the glass/plastic/glass laminates of Saunders and Snedeker, et al uses glass on only one of the two outer faces. As applied to face masks or windshields, the exterior surface is the surface primarily exposed to weathering and scratching. Accordingly, a laminate of glass, polyurethane, and polycarbonate or polyacrylate can be coated on the inner face with a Mylar or polyester layer, rather than a second layer of the heavier glass, to decrease weight. Inasmuch as the inside face is less exposed to the elements, the nonsymmetrical arrangement is adequately resistant to scratching and chemical deterioration. Hardening of the surface of the inner plastic face can also be chemically accomplished to resist scratching and weathering. Such treatments are known in the art.
A major difficulty encountered in fabricating the nonsymmetrical multi-layer laminates of the prior art resides in the need to cure or laminate the plastic inter-layers at elevated temperatures. Relatively high temperatures are required to achieve adequate bonding between the glass and plastic, and between the multiple plastic layers which make up the desired laminate. The large difference in coefficient of thermal expansion between glass and plastic introduces substantial shear stress into the curved laminate during cycling of temperature during lamination or curing, which stress is especially troublesome during the cool-down phase. These shear stresses often result in delamination or even breaking and crazing of the glass layer. The losses experienced in production of symmetrical glass/plastic/glass sandwiches are compounded in production of nonsymmetrical glass/plastic laminates. In practice, the producer of safety glass experiences higher costs due to poor product yield caused by delamination or shattering of a certain percentage of the finished parts. Such higher costs are passed on to the consumers and preclude the use of safety glass laminates in many applications where cost is crucial. As a result, short lived all-plastic face masks, goggles and other protective gear are often used where longer lived glass-plastic laminates would provide a more durable product.
The present invention teaches the use of a previously formed aliphatic polyether polyurethane sheet for the interlayer between the glass and other plastic layers. Such use allows curing at lower temperatures of about 150.degree.-230.degree. F. (66.degree.-110.degree. C.), and preferably at about 180.degree.-220.degree. F. (82.degree.-104.degree. C.). Pressure is combined with the temperature to assist bonding and to minimize shear stresses upon cooling down.
The lamination can be conveniently carried out in one step or in two steps. Where thicker glass or large pieces of glass are to be used, it is presently preferred that the aliphatic polyurethane sheet be first laminated to the glass. At a temperature of about 210.degree. F. (99.degree. C.), a Teflon or Nylon pressure plate is brought against the polyurethane over the glass, then cooled. Next the polycarbonate or polyacrylate sheet is placed against the polyurethane side of the newly formed glass-polyurethane laminate, centered and pressurized to about 150 psi. The temperature is brought up to about 160.degree. F. (71.degree. C.) for about 60 minutes, then the completed laminate is cooled. It will be appreciated that this method avoids subjecting the polycarbonate or polyacrylate sublayer to the full temperature of about 200.degree. F. at which the bonding of the polyurethane and glass is preferably accomplished. Alternatively, polyurethane with an aliphatic polyether backbone can be formed in place as the inter-layer using a curing accelerator to produce a low temperature cure. Such a process results in improved hydrolytic stability, resistance to yellowing and improved product yield due to decreased incidence of delamination and shattering.